The Diabetes & Womens Health Study, based on an innovative hybrid design combining new prospective data collection with historical data, aims to understand pathways and determinants underlying the progression from gestational diabetes (GDM) to type 2 diabetes (T2DM) and complications such as cardiovascular diseases. GDM is a common pregnancy complication. Women who develop impaired glucose tolerance and/or GDM in pregnancy are at substantially increased risk for T2DM in the years following pregnancy. Determinants underlying the transition from GDM to T2DM are not well studied and available studies are limited. There is limited information about the genetic and environmental factors that impact this transition in particular. Further, there is limited research aimed at following women with GDM long term through later adulthood for the development of T2DM and co-morbidities. These critical data gaps serve as the impetus for this study with the overall goal of investigating genetic factors and their interactions with risk factors amenable to clinical or public health intervention in relation to the transition of GDM to T2DM. Data collection for this study builds upon two large existing cohorts: the Nurses' Health Study II (NHS-II) and the Danish National Birth Cohort (DNBC). Study recruitment began in March of 2012. Outstanding progress has been made since then with enrollment reaching 96% of the study goal. The first cycle data collection was successfully completed in April 2014. In October 2014, the second cycle data collection was launched. It is expected that all the DWH Study field data collection will be completed in September 2016. Measurement of genetic data and development of a certified data set from the Data Coordinating Center will follow. The overall design of the study was published in 2014 (Zhang et al. Acta Obstetricia et Gynecologica Scandinavica). Briefly, key medical, reproductive, anthropometric, and multi-level environmental factors (e.g. diet, lifestyle, socio-behavior) were collected using standardized questionnaires in both the US and Denmark. Furthermore, women reported information on their offspring from their GDM pregnancy. In addition, timed biospecimens, including fasting blood, first morning urine, saliva, and toenails were collected. Further study details are available at the Studys website (www.dwhstudy.org). One unique advantage of the hybrid design is the availability of historical data which allows investigators to address relevant research questions and publish findings while pursuing new data collection prospectively. Comprehensive historical datasets with data longitudinally collected over the past 15-20 years from both the NHS II study and DNBC among more than 7500 individuals (with more than 30,000 variables available from each individual) were delivered to the DWH study's data coordinator center (DCC) and transferred to the NICHD in March, 2012. Based on the longitudinal historical datasets, more than 15 analyses and manuscripts have been planned. A couple of manuscripts have been published in high impact medical journals and a few more are in press or under review.